Friday, May 9, 2008

C Shell commmands

The C shell provides the following built-in commands:

# Marks a command.
alias Displays alias.
bg Resumes job in the background.
break Resumes execution after the loop.
breaksw Breaks from a switch command; resumes after the endsw command.
case Defines a label in a switch command.
cd Changes directory.
chdir Changes directory, same as cd.
continue Continues a loop.
default Specifies the default case in a switch.
dirs Displays the directory stack.
echo Writes arguments to the standard output of the shell.
eval Evaluates a command.
exec Executes the command in the current shell.
exit Exits the shell.
fg Brings a job in the foreground.
foreach Specifies a looping control statement and execute a sequence of commands until reaching an end command.
glob Writes arguments to the standard output of the shell, like the echo command, but without the new line.
goto Continues execution after the specified label.
hashstat Displays hash table statistics.
history Displays the history list.
if Executes a command if condition met.
jobs Lists active jobs.
kill Sends a signal to a process. term (terminate) is the default signal.
limit
Sets or list system resource limits.
login Logs on.
logout Logs out.
nice Changes the priority of commands run in the shell.
nohup Ignores the hangup signal.
notify
Notifies the user about changes in job status.
onintr Tells the shell what to do on interrupt.
popd Pops the top directory off the directory stack and changes to the new top directory.
pushd Exchanges the top two elements of the directory stack.
rehash Re-computes the hash table of the contents of the directories in the path shell variable.
repeat
Repeats the execution of a command.
set Displays or set the value of a shell variable.
setenv Sets environment variables.
shift Shifts shell arguments.
source Reads commands from a script.
stop
Stops a background job.
suspend Stops the current shell.
switch Starts a switch.
time Displays the time used to execute commands.
umask Shows or set file permissions.
unalias
Removes command alias.
unhash Disables the internal hash table.
unlimit Removes limitations on system Resource.
unset Deletes shell variables.
unsetenv Deletes environment variables.
wait Waits for background jobs to complete.
while …end
Executes the commands between the while and matching end statements repeatedly.
@ Displays or set the values of all the shell variables.




The Linux/Unix shell refers to a special program that allows you to interact with it by entering certain commands from the keyboard; the shell will execute the commands and display its output on the monitor. The environment of interaction is text-based (unlike the GUI-based interaction we have been using in the previous chapters) and since it is command-oriented this type of interface is termed Command Line interface or CLI. Before the advent of GUI-based computing environments, the CLI was the only way that one can interact and access a computer system.

Up until now, there was never a need to type commands into a shell; and with the modernisation and creation of a lot of newer GUI-based tools, the shell is becoming increasingly un-required to perform many tasks. But that said, the shell is a very powerful place, and a lot is achieved through it.

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A lot of the front-end GUI methods of doing things have similar ways and means to get done with using the shell. Professional Linux and UNIX users find the shell very powerful, and an introduction to at least the basic shell usage is useful.--
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